This invention relates to electrical apparatus and is particularly, but not exclusively, related to housings for microwave circuits. The term microwave includes circuits operating at or above a frequency of 1 GHz and may cover millimetric, quasi-optical and optical frequencies.
It is convenient to house an electrical circuit in a housing. As well as providing protection against environmental and other influences the housing can provide an electrical function. Electrically conducting metal boxes are used as housings and serve to screen against leakage of electrical signals. This is particularly important for a circuit operating at microwave frequencies where electrical signals are prone to leak from the circuit. A conductive housing may also provide a conductive ground plane or a conductive path. In addition it is convenient for a housing to be thermally conductive in order to remove heat from the circuit.
Conductive housings are conventionally formed out of sheet metal, such as aluminium, brass or stainless steel, or machined out of solid metal. Aluminium is preferred because of its ease of machining, low cost and low weight.
Circuits are often fabricated on thin substrates of ceramic type materials including aluminium oxide, boron nitride and sapphire or on semiconductor materials such as silicon, gallium arsenide, CdHgTe or InP. It is important that the substrate is in intimate contact with a thermally conductive mounting surface (which is usually flat) in order to remove heat efficiently from the substrate and thus from the circuit. It is also important that good electrical contact is achieved between the substrate and the mounting surface. Usually the face of the substrate in contact with the mounting surface is provided with a conductive layer and bonded with a conductive adhesive. The substrates generally have a low coefficient of thermal expansion (CTE) and metal housings in which they are housed have a relatively high CTE. Therefore direct mounting of such a substrate directly onto part of a metal housing is not advisable since over the temperature range of operation, differential thermal expansion may damage the substrate. This is particularly the case for a microwave circuit because it may be relatively large, for example in the region of 10 cm long.
Materials which can be used as mounting surfaces are available which have a low CTE over a limited range of temperatures. These materials are usually metal alloys. An example is a commercially available material called Kovar (Fe-29Ni-17Co wt%). Although these materials are satisfactory for small circuits, they are less so for larger circuits since generally they are expensive. Furthermore, they have high densities, relatively poor thermal conductivity and they do not machine readily.
Another type of material having a low CTE is a metal matrix composite (MMC) material. This comprises a metal matrix or binder having in it a filler of a material different to the matrix, often in the form of crystals in the microstructure of the alloy material. MMC material is relatively lightweight, has high thermal conductivity and low CTE. Furthermore it is relatively stiff. This is advantageous because it resists flexing under vibration and thus protects the circuit from damage. However MMC materials are generally difficult to machine.
Difficulty in machining such low CTE materials increases as the size of the housings increase. Simple single compartment small housings are easier to make than large housings having multiple, relatively deep, compartments.
It is an object of the present invention to provide an improved housing for electrical circuits.
According to a first aspect the present invention provides a housing for an electrical circuit comprising a plurality of parts joined together by diffusion soldering in which at least one hole passing from the inside of a compartment in the housing to the outside of the compartment is formed at the junction of at least two of the parts.
Preferably a housing comprises a plurality of compartments each of which is adapted to receive at least one circuit. The or each circuit may be sealed in a compartment by a lid. The or each hole may pass from the inside of the compartment to the inside of another compartment or from the inside of the compartment to the outside of the housing.
Using diffusion soldering is advantageous because it means that there can be a series of steps in assembling the housing. It permits assembly in a number of separate stages without previously made joints remelting. It is a relatively low temperature process which minimizes distortion of the base by heat or differential contraction following the joining operation. This is important because there must be good thermal and electrical contact as discussed above.
Preferably the housing is conductive. The housing may be coated with a high conductivity material such as silver. This is advantageous for high frequency applications. A coating may be applied to individual pieces before joining, or to a complete assembly.
Preferably one or more of the plurality of parts comprises MMC material. Preferably the MMC material comprises a matrix such as aluminium or aluminium/copper or aluminium/silicon. Preferably the metal matrix composite comprises a dispersion of crystals in the matrix. The dispersion may comprise silicon and/or silicon carbide. Preferably the ratio by volume of matrix material and dispersion material is 1:1. Most preferably there is more dispersion material than matrix material by volume.
Preferably the housing is fabricated by assembling together one or more walls and a base. Conveniently the walls may comprise an external outer wall divided by one or more internal walls into a plurality of compartments. The base may be of a low CTE material such as Kovar. Preferably it is of MMC material. Preferably the walls are of MMC material. Alternatively the walls may be of another material such as Kovar, aluminium, aluminium alloy, copper alloy including copper/tungsten or steel.
Preferably at least one wall and the base are of different materials. Any combination of materials may be used as long as they have appropriate electrical properties and their CTEs are not sufficiently different so as to lead to stresses caused by differential thermal expansion or contraction being larger than the strength of the joints over the temperature range of use. An advantage of making parts of the housing from different materials is that port of the housing which is to be of MMC material can be chosen to be in a shape which is relatively easy to machine or fabricate whilst more complex shapes, for example the walls, may be formed of material which may easily be pressed, punched, bent or otherwise formed into shape. As a result, parts of the housing not needing to be of low CTE may be made from cheaper materials. Furthermore, they may be fabricated to be thinner and lighter. One such embodiment may be an MMC material base and a peripheral wall formed from aluminium.
The circuit may be joined to the base by a suitable conductive adhesive or they may be diffusion soldered together. The circuit may be attached directly to the base or attached to an intermediate plate of MMC material or another low CTE material which is attached in turn to the base.
The term circuit includes a circuit mounted on a substrate. Preferably the substrate is in contact with the base.
Preferably the or each hole is adapted to receive a feedthrough.
A further advantage of joining initially separate walls to the base is that feedthroughs for conductors passing through the walls may be located very close to the base. The closeness of holes to a base in monolithic housings is determined by tool clearance inside a compartment and above the base. Conveniently, rather than holes being drilled, cut-outs or notches at an edge of a wall may be machined which form holes when the walls and base are joined.
According to a second aspect the present invention provides a housed circuit housed in a housing in accordance with the first aspect of the invention.
Preferably the electrical circuit is a microwave circuit.